The present invention relates to a method and apparatus for acquiring physical information, and a method for manufacturing a semiconductor device including an array of a plurality of unit components, for detecting a physical quantity distribution. More specifically, the invention relates to a signal acquisition technique suitably applied to a solid-state imaging device using a semiconductor device for detecting a physical quantity distribution, the semiconductor device including an array of a plurality of unit components sensitive to electromagnetic waves input from the outside, e.g., light and radiations, so that a physical quantity distribution converted into electric signals by the unit components may be read out as the electric signals. In particular, the invention relates to an imaging device which permits imaging with a wavelength component (for example, infrared light) other than visible light.
The invention also relates to a light-receiving device and a solid-state imaging device each including photoelectric transducers formed in a semiconductor layer of silicone, a compound semiconductor, or the like, and methods for manufacturing the respective devices.
Semiconductor devices for detecting a physical quantity distribution are used in various fields, the semiconductor devices each including a line or matrix array of a plurality of unit components (for example, pixels) sensitive to changes in physical quantities such as electromagnetic waves input from the outside, such as light and radiations.
For example, in the field of video apparatuses, CCD (Charge Coupled Device), MOS (Metal Oxide Semiconductor), or CMOS (Complementary Metal Oxide Semiconductor) solid-state imaging devices for detecting changes of light (an example of electromagnetic waves) as an example of physical quantities are used. In these devices, a physical quantity distribution converted into electric signals by unit components (e.g., pixels in a solid-state imaging device) is read out as the electric signals.
For example, a solid-state imaging device detects an electromagnetic wave input from the outside, such as light or a radiation, using photodiodes serving as photoelectric transducers (light-receiving device; photosensors) provided in an imaging portion (pixel portion) of the device, thereby generating and accumulating signal charges. The accumulated signal charges (photoelectrons) are read out as image information.
In recent years, structures for visible light imaging and infrared light imaging have been proposed (refer to, for example, Japanese Unexamined Patent Application Publication Nos. 2004-103964, 10-210486, 2002-369049, 6-121325, 9-166493, 9-130678, and 2002-142228). For example, an infrared luminous point is previously prepared so that the position of the infrared luminous point in a visible light image may be detected by tracking the infrared luminous point. In addition, for example, even in the night without visible light, a clear image may be obtained by imaging with infrared irradiation. Furthermore, the sensitivity may be improved by taking in infrared light in addition to visible light.
The structure disclosed in Japanese Unexamined Patent Application Publication No. 2004-103964 is a single plate type using changes in absorption coefficient with wavelength in the depth direction of a semiconductor.
The structures disclosed in Japanese Unexamined Patent Application Publication Nos. 10-210486, 2002-369049, and 6-121325 are each a multi-plate type using a wavelength resolving optical system including a wavelength separation mirror and prism as an input optical system so that visible light and infrared light are received by respective imaging devices.
The structure disclosed in Japanese Unexamined Patent Application Publication No. 9-166493 is a single-plate type using a rotating wavelength resolving optical system as an input optical system so that visible light and infrared light are received by the same imaging device. For example, when an infrared cut filter is inserted/extracted by a rotating mechanism, with the infrared cut filter inserted, a visible color image is output without being influenced by near-infrared light and infrared light, while with the infrared cut filter extracted, an image with light intensity including visible light intensity and near-infrared light intensity is output.
The structure disclosed in Japanese Unexamined Patent Application Publication No. 9-130678 uses a diaphragm optical system having a wavelength resolving function as an input optical system so that visible light and infrared light are received by the same imaging device.
The structure disclosed in Japanese Unexamined Patent Application Publication No. 2002-142228 includes an imaging device sensitive to visible light and near-infrared light, in which four types of color filters having respective filter characteristics are regularly disposed on pixels, and a visible color image and a near-infrared light image are independently determined by matrix-calculation of the outputs of the respective pixels on which the four types of color filters are disposed.
A solid-state imaging device includes photoelectric transducers formed in a semiconductor layer.
Therefore, the solid-state imaging device has the problem of generating a so-called dark current due to the surface level of the semiconductor layer in which the photoelectric transducers are formed.
As shown in a potential diagram of FIG. 60A, the dark current is mainly generated by the phenomenon that electrons trapped at the surface level are thermally exited to a conduction band and thus moved to the n-type semiconductor region of a photodiode constituting each photoelectric transducer by the electric field of a surface depletion layer.
For example, in a semiconductor layer composed of silicon, the band gap is 1.1 eV, and the surface level (and the Fermi level) is present at a position where the band gap is divided at 2:1 by the Bardeen limit.
Therefore, the potential barrier against the electrons trapped at the surface level is 0.7 eV.
Therefore, in order to decrease the dark current due to the surface level, a method of forming a p+ layer on a surface of a photodiode is used (refer to, for example, Japanese Unexamined Patent Application Publication No. 2002-252342, FIG. 5).
This method suppresses the dark current to some extent.
Namely, as shown in a potential diagram of FIG. 60B, the potential barrier against the electrons trapped at the surface level becomes 1.0 eV due to the presence of the p+ layer. In other words, the potential barrier is increased by about 0.3 eV as compared with a case in which the p+ layer is absent, and thus the number of electrons thermally exited may be decreased to decrease the dark current.
When a p+ layer is provided on a surface of a silicon substrate, the quantity of the dark current at room temperature (T=300K) estimated from a Fermi-Dirac distribution function is decreased by four digits, as compared with a case in which the p+ layer is absent.
A Fermi-Dirac distribution function is represented by the following equation 10:
                              f          ⁡                      (                          E              ,              T                        )                          =                  1                      1            +                          ⅇ                                                E                  -                                      E                    F                                                  kT                                                                        Equation        ⁢                                  ⁢        10            
wherein E is energy, EF is Fermi energy, T is an absolute temperature, k is the Boltzmann constant, e is a natural logarithm, and E-EF corresponds to the magnitude of a potential barrier.
FIGS. 53A and 53B are drawings illustrating the structure of a sensor disclosed in Japanese Unexamined Patent Application Publication No. 2004-103964, in which FIG. 53A is a drawing showing the light absorption spectral characteristics of semiconductor layers, and FIG. 53B is a schematic drawing showing a sectional structure of a device.
In this structure, the light absorption coefficient of a Si (silicon) semiconductor decreases in the order of blue, green, red, and infrared light, as shown in FIG. 53A. Namely, with respect to blue light, green light, red light, and infrared light contained in incident light L1, by using the position dependency of wavelength in the depth direction of a semiconductor, layers for detecting visible light (blue, green, and red) and infrared light, respectively, are provided in order in the depth direction from a surface of the Si semiconductor as shown in FIG. 53B.
However, in the structure disclosed in Japanese Unexamined Patent Application Publication No. 2004-103964 which utilizes variations in absorption coefficient with wavelengths, red light and green light are absorbed by a layer for detecting blue light to some extent when being passed through this layer and are thus detected as blue light although the quantity of theoretically detectable light is not decreased. Therefore, even when there is no original signal of blue light, signals of green light and red light are entered to enter a signal of blue light, thereby producing an alias and thus failing to achieve sufficient color reproducibility.
In order to avoid this problem, correction is preferably performed by signal calculation processing for the entire of the three primary colors, and thus a circuit for calculation is separately provided. Accordingly, a circuit configuration is complicated and increased in scale, and the cost is also increased. Furthermore, for example, when one of the three primary colors is saturated, the original value of the saturated light is not determined to cause error in the calculation. As a result, signals are processed so as to produce a color different from the original color.
As shown in FIG. 53A, most semiconductors have absorption sensitivity to infrared light. Therefore, for example, in a solid-state imaging device (image sensor) using a Si semiconductor, an infrared cut filter made of glass is preferably inserted as an example of subtractive color filters in front of the sensor.
Therefore, in order to take an image by receiving only infrared light or visible light and infrared light as signals, preferably, the infrared cut filter is removed or the cut ratio of infrared light is decreased.
However, in such a case, infrared light is mixed with visible light and incident on a photoelectric transducer, thereby producing a visible light image with a color tone different from the original tone. It may be thus difficult to separately produce a proper visible light image and a proper image of infrared light alone (or mixture of infrared light and visible light) at the same time.
Apart from the above-described problem, in ordinary solid-state imaging devices, visible light is also cut to some extent by using the infrared cut filter, thereby decreasing sensitivity. The cost is also increased by using the infrared cut filter.
In the structures disclosed in Japanese Unexamined Patent Application Publication Nos. 10-210486, 2002-369049, and 6-121325, the input optical system is increased in scale by the wavelength resolution optical system including a mirror and a prism for wavelength separation.
In the structure of Japanese Unexamined Patent Application Publication No. 9-166493, a device is increased in scale by the infrared cut filter insertion/extraction mechanism, and the infrared cut filter is not automatically operated.
In the structure of Japanese Unexamined Patent Application Publication No. 9-130678, a device is increased in scale by the diaphragm optical system having a wavelength resolving function. In addition, although both an infrared light image and a visible light image may be simultaneously obtained, only electric composite signals of the visible light image and the infrared light image are output from the image sensor, thereby failing to output only the visible light image or only the infrared light image.
On the other hand, in the structure of Japanese Unexamined Patent Application Publication No. 2002-142228, wavelength separation is performed using the four types of color filters. Therefore, this structure has a problem with arithmetic processing but not have the problem of Japanese Unexamined Patent Application Publication Nos. 10-210486, 2002-369049, 6-121325, 9-166493, and 9-130678 in which the input optical system is increased in scale. Namely, in the structure of Unexamined Patent Application Publication No. 2002-142228, a visible color image and a near-infrared light image are independently determined by matrix operation of the outputs of the pixels on which the four types of color filters having respective filter characteristics are respectively disposed, and thus a visible light image and an infrared light image may be separately and simultaneously output. However, even when a visible light image is obtained, arithmetic processing is performed between visible light and infrared light components, thereby significantly increasing arithmetic processing as a whole.